A method of calculating the output factors of arbitrarily shaped electron fields

Abstract

It has been well demonstrated in the past that the dose output of an electron beam is highly dependent upon the design of the collimation system, the beam energy, and the field dimensions. Various approaches have been taken to calculate the output factor of an arbitrary rectangular beam. These methods, however, cannot be readily extended to accurately calculate the output factor of an irregular field. In the work, the output factor of an arbitrarily shaped electron field is calculated as the product of three terms: a weighting factor which is explicitly derived from a discrete electron pencil beam model, the known output factor of the regular electron field defined by an applicator or a set of trimmers from which the irregular field is formed, and a machine-dependent term which models the scatter contribution from the shielding. For the machine and fields of clinical interest discussed here, it was found that this latter term could be neglected; this may not be true in general. Results from this calculation are compared to measured data.